Many site issues can be caused by corrupt cookies or cache. In order to try to fix these problems, the first step is to clear both cookies and the cache.
Note: This will temporarily log you out of all sites you're logged in to.
To clear cache and cookies do the following:
Go to Firefox > History > Clear recent history or (if no Firefox button is shown) go to Tools > Clear recent history.
Under "Time range to clear", select "Everything".
Now, click the arrow next to Details to toggle the Details list active.
From the details list, check Cache and Cookies and uncheck everything else.